The desire to reduce our dependence on foreign oil and to reduce pollution has lead to the pursuit of several cleaner energy production technologies. Fuel cells produce electricity more efficiently and more cleanly than existing hydrocarbon fueled energy production methods that use simple combustion. Before optimum commercialization of fuel cells may be realized, they must achieve a higher level of performance. Goals identified for a commercial fuel cell include high specific power (>1.0 kW/kg), high energy efficiency (40-60%), low cost (<$200/kW), fast response, high durability (>5000 hrs), and good fuel flexibility.
It has been contemplated that in the future fuel cells may run primarily on hydrogen produced from reforming renewable biofuels. However, in order to effect timely commercialization of fuel cells and facilitate a transition from existing fossil fuels to biofuels and their derivatives, fuel cells must be capable of consuming more traditional hydrocarbons found in fossil fuels, for example natural gas, liquid petroleum gas, petroleum, and other hydrocarbons. Most existing fuel cells, however, operate most efficiently when consuming syngas, i.e. a combination of mainly hydrogen, carbon monoxide and some carbon dioxide. Thus, long chain and other hydrocarbons found in fossil fuels, e.g. petroleum, must be reformed into syngas before they may be efficiently used as fuel in a fuel cell.
Solid oxide fuel cells (SOFCs), and particularly low temperature SOFCs operating at 450-650° C., are particularly attractive possible fuel cell designs for generating electricity. A lower operating temperature requires less complexity in design and fabrication, enhanced flexibility in materials selection, enhanced reliability and durability, less insulation, higher thermal efficiency, and reduced stack volume and mass. However, development of a compatible reformer operating at lower temperatures for high hydrocarbon fuels proves to be a very challenging task.
Two reforming schemes have been implemented in SOFC systems: external reforming and internal reforming (IR). In the case of internal reforming, the reformation of the hydrocarbon fuels takes place in the SOFC anode reaction chamber. The scheme of internal reforming is further categorized into indirect internal reforming (IIR) when the reforming reactions are separated spatially from the electrochemical reactions and direct internal reforming (DIR) when the reforming reactions take place on SOFC anode. Use of an IR schemes eliminates the external reformer as a subsystem, reduces the total size and mass, facilitates heat transfer, and enhances the thermal efficiency.
The reforming reaction, while thermodynamically favored, is inherently slow, and non-selective for the desired products. Various catalysts have been developed, but are still insufficient to reform hydrocarbons at the speed and temperature desired for SOFCs.
Enhancement of intrinsic catalytic activity requires improvement of the catalyst material itself. Most catalysts have three components: an active phase, a promoter and a support. Precious metals or alloys are commonly used as active phases. Ni and Ni alloys may be used as non-precious metal active phases. However, they require both a high operating temperature and small quantities of precious metals to reduce coking. Most of existing methods of reforming require high temperatures (>700° C.) and a large amount of oxygen or steam.
Common catalyst supports such as alumina provide a catalyst carrier that prevents sintering or ripening of the active phases and ensures a stable active surface. More recently, oxygen ion conducting materials, such as ceria and zirconia, have been used as catalyst supports. Catalysts on non-stoichiometric ceria exhibit increased oxidation catalysis due to a metal-support interaction. The ceria support material provides a stable surface for oxygen species in the oxidation reaction. Consumed oxygen in the ceria lattice is replaced by transported of oxygen ions from oxidizing molecules to the lattice. The reforming reaction typically involves at least two reactant molecules and takes place at active sites on the catalyst only a few nanometers wide.
Electrochemical promotion of catalysis in the solid-state electrochemical systems with an external power source can accelerate reaction kinetics remarkably at temperatures as low as 350° C. Electrochemical promotion is functionally identical to classical promotion; i.e., it is catalysis in the presence of a controllable electric double layer at the metal/gas interface. The mechanism of electrochemical promotion is thought be due mainly to production of short-lived sacrificial promoters, such as O2−, O22−, O−, or most likely O2−, which are continuously supplied to the catalyst/gas interface via electrochemically controlled transport including surface diffusion from the solid electrolyte support. It is also called non-Faradaic electrochemical modification of catalytic activity (NEMCA). The rate enhancement ratio is defined by
                    ρ        =                  r                      r            0                                              (        1        )            and the enhancement factor (electrochemical promotion efficiency) by
                    λ        =                              r            -                          r              0                                            I                          2              ⁢              F                                                          (        2        )            It has been demonstrated that the electrochemical promotion of catalytic reforming can increase promotion efficiency be more than 2 orders of magnitude, λ>>1, even at very low current density (˜1 mA cm−2). Unfortunately, the electrochemical promotion can only be enabled via an external power source or short-circuiting the anode and cathode of an electrochemical cell, which is equivalent to a single-chamber solid oxide fuel cell (SC-SOFC). This is neither realistic nor easy to accomplish.
It is also desirable to minimize the production of nitrogen oxides (NOx) during combustion or other energy production using fossil fuels. Electrochemical promotion can accelerate nitrogen oxide reduction in the presence of a reducing agent and oxygen by several orders of magnitude (×104) even at a low current density, such as about 5 mA/cm2 and at low temperatures such as about 300° C. to about 650° C., but this requires use of precious metal catalysts. Precious metals have also consistently been required in order to prevent coking of the catalyst material.
It is therefore desirable to provide a system and method for reforming fossil fuels into syngas usable by a low temperature solid oxide fuel cell. It is also desirable to provide a strong catalyst for hydrocarbon reforming that does not require an external source of electric current. It is also desirable to provide a system and method for catalyzing the reforming fossil fuels into compound usable by a solid oxide fuel cell at a low temperature without an external electric current supply or high pressure oxygen or steam. It is also desirable to provide a system and method for reforming fossil fuels that minimizes the production of nitrogen oxides and minimizes coking of the catalyst.